The Earth's magnetic field, also known as a geomagnetic field, is a magnetic field that extends from the Earth's interior to a region above the Earth that is exposed to solar wind (i.e., a stream of charged particles that are exposed from the Sun). Magnetic fields are vector quantities that are characterized by both strength and direction. The magnitude of the Earth's magnetic field at the Earth's surface can range from 20 to 80 microtesla (mT). This range is equal to 0.20 to 0.80 gauss or 20,000 to 80,000 nanotesla (nT). The magnitude of the Earth's magnetic field can vary depending on location.
Magnetic field variations due to magnetic anomalies can be in the picotesla (pT) range. In general, magnetic anomalies are local variations in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. For example, magnetic surveys over the oceans have revealed a characteristic pattern of anomalies around mid-ocean ridges. These patterns involve a series of positive and negative anomalies in the intensity of the magnetic field, which form stripes running parallel to each ridge. The source of these magnetic anomalies can be from magnetization carried by titanomagnetite minerals in basalt and gabbros, which are magnetized when ocean crust is formed at the ridge.
The Earth's magnetic field and magnetic field anomalies can be measured using a measurement instrument referred to as a magnetometer. Examples of magnetometer include vector magnetometers and scalar magnetometers. Vector magnetometers can measure vector components of the Earth's magnetic field. Scalar magnetometers can measure the magnitude of a vector magnetic field. Magnetometers can also be classified based on their intended use. For example, stationary magnetometers can be installed to a fixed position and measurements are taken when the magnetometer is stationary. On the other hand, portable magnetometers are usable while in motion and can be transported in a moving vehicle.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.